This invention relates to subtractive color hot melt ink reflection images formed on opaque substrates such as paper and to methods for making such images.
Hot melt inks are used in thermal transfer printers and in certain ink jet printers. The characteristics of these inks is that they are solid at room temperature, are liquefied by heating for application, and are resolidified by cooling on the printed substrate. Colored reflection prints may be made by applying successive layers containing different subtractive colors onto an opaque substrate such as paper. In contrast to conventional additive color printing processes in which small opaque colored dots of different color are printed in side-by-side relation and the eye of the observer integrates the combined effect of the light reflected from different colored dots, subtractive color printing utilizes transparent superimposed layers of different colors which act successively on the same light rays to produce reflected light rays having a desired color.
Subtractive color hot melt ink reflection images may be prepared by applying successive different-colored layers comprising individual drops of the ink by an ink jet system, for example, or by transferring successive ink layers of uniform thickness to the substrate if a thermal transfer process is used. In each case, the colored ink image printed on the substrate is produced by a single ink layer or by the combined effect of successive layers through which light is reflected from the opaque substrate to the eye of the observer.
Heretofore, it has been understood that colored ink projection images printed on a transparent base to provide a projection transparency must be substantially transparent, i.e., without significant quantities of impurities, crystalline content or frosted surfaces, and nonrefractive, i.e., without significant dispersion of light by lenslets formed by individual drops of ink on the surface of the transparency, in order to project a remote image of the transparency in full color with a projection lens. Such difficulties in the preparation of transparencies containing hot melt color ink images may be overcome, for example, by using the procedures described in the Fulton et al. U.S. Pat. No. 4,873,134. According to that patent, the problems associated with hot melt ink color transparencies can be eliminated by maintaining the hot melt ink image at a temperature above its melting point for a selected time, thereby permitting the hot melt ink spots to spread so as to reduce the curvature of the lenslets formed by the spots, and then cooling the ink rapidly to inhibit crystallization and frosting of the ink, which could scatter the light transmitted through the transparency away from the path to the projection lens and detract from the projected image produced by a projection lens.
Moreover, in the Spehrley, Jr., et al. U.S. Pat. No. 4,751,528, a hot melt ink jet system is described in which the temperature of the platen supporting a paper substrate to which ink is supplied by an ink jet is controlled at a selected level which is related to the melting point of the ink so as to permit the ink to spread to a desired extent within the paper before solidification. Heretofore, however, the effect of crystallization and frosting of hot melt ink applied to an opaque substrate such as paper to produce subtractive color reflection images has not been considered and no attempt has been made to reduce such crystallization or surface frosting of ink spots in color images on paper.
For example, as described in Billmeyer and Saltzman (Principles of Color Technology, 1981), Kubelka and Munk ("Ein Beitrag zur Optik der Farbanstriche", Z. tech. Phys. v12, pp. 593-601, 1931) advanced a theory to predict the color created by applying ink to paper. The theory uses two functions to describe each ink on paper: an absorption coefficient and a scattering coefficient, both of which are functions of wavelength. The general printing industry usually makes a major simplification to the Kubelka-Munk theory: it assumes that the scattering component of the Kubelka-Munk theory is due to scattering by the paper, and the absorption component is due to the ink. Moreover, inks in general use in the printing industry do not have a crystallinity problem, or no such problem has been recognized. In one case in which the Kubelka-Munk theory was used to solve a practical problem, scattering of light was a significant contributor to poor image color, but the scattering was attributed to the paper substrate.